Concrete is a composite material manufactured by mixing together portland cement, fine aggregate (sand), coarse aggregate (gravel or crush rock), and water. Other potential ingredients include supplementary cementitious materials—such as fly ash, slag cement, and silica fume—and chemical admixtures—such as water reducers, set accelerators, set retarders, and air entraining agents.
The addition of water to portland cement results in a series of chemical reactions, known as hydration, ultimately resulting in hardened cement paste that binds together coarse and fine aggregate to form hardened concrete. Some water is needed for hydration; however, too much water damages concrete hardened properties such as strength and durability. Increasing the water content also improves the workability and flowability of concrete, often measured with the slump test. Concrete is designed with specific water content to achieve the desired workability and hardened properties needed for the application. If too little water is added, the concrete is stiff and difficult to pour and finish. If too much water is added, the strength and durability are poor.
For ready mix concrete, the individual ingredients are batched in a central plant and transported by truck to the jobsite. The ingredients may be first mixed in a stationary mixer at the plant and then added to the truck mixing drum (or “concrete drum”) or they may be added directly to the truck mixing drum where all mixing takes place.
Water is added to concrete from multiple sources. During batching, water is introduced directly into the concrete drum along with the other dry ingredients. In addition, aggregates contain some moisture, which must be accounted for when determining total water content. It is common to batch less water than the design, such that water can be added later if the slump is less than desired. Once the truck is loaded, it typically moves to the “slump rack” (also known as the “wash rack”) in the plant yard. Here, the driver washes off any dust that collects on the outside of the truck during batching. He washes down into the concrete drum any material that collects on the top of the inside of the concrete drum and on the hopper for loading the truck. In addition, the driver checks the slump and may add water if the slump is less than target. Unexpected variation in batching or material quality may result in the slump being less than desired and necessitate a slump adjustment at the slump rack. Once the truck leaves the plant, the driver may add more water during transit or on the jobsite before or after pouring to increase the slump. This may be necessary because slump typically decreases over time.
It is essential that all water be measured and compared to the design amount. Water in the form of aggregate moisture is measured with aggregate moisture meters (e.g. microwave or resistivity meters) or by collecting a sample, drying the sample to remove all water, and recording the mass of water evaporated. Water added during batching is measured by weight or with a flow meter. Trucks are equipped with water tanks for adding water to the concrete drum in transit or at the jobsite. The amount added may be measured with a flow meter on the truck or a graduated sight tube on the water tank.
Any water added in the plant after batching is difficult to measure. Truck drivers typically use a hose from the plant, which is external to the truck, to wash the truck and add water to the concrete drum. This hose is typically not metered. Even if it were metered, the hose is common for all trucks, so it is difficult to track which truck the water went to and whether the water was added to the concrete drum or used to wash the exterior of the truck.
The water added from the truck tank may also be used for purposes other than adjusting the concrete. For example, some water is used to clean the chutes and tools.
It is also important to know when water was added, relative to the delivery cycle. For example, if water is added after pouring has started, the purchaser of the concrete is typically responsible for any negative consequences of this water addition. However, the ready mix concrete supplier is typically responsible for the consequences of any water added prior to delivery on the jobsite.
Therefore, what is needed is an apparatus, system, and method to measure all water added to the concrete drum and to exclude any water coming from the truck tank but not added to the concrete drum. It is also important to achieve this goal without increasing the amount of time to deliver a load of concrete.